how to find the difference b/w to dates stored in a variable in shell script ( KSH shell )

Question

In hp-ux as there is no GNU date available, i.e the -d and -s options of the date command are not available. How do I find the difference between two given dates in days (as we do in Solaris or bash shell)?

Answer 1

There are actually many solutions to be found via Google, but here is one that I just stumbled across and like for its brevity. You would use its ansi_dn function on both dates and calculate the difference.

diff_dates.sh:

#!/bin/ksh

. ./ansi_dn.sh

date_to="$1"
date_from="$2"

echo $((
      $(ansi_dn \
        $(echo "${date_to}"   | cut -d/ -f3) \
        $(echo "${date_to}"   | cut -d/ -f2) \
        $(echo "${date_to}"   | cut -d/ -f1) \
      )
    - $(ansi_dn \
        $(echo "${date_from}" | cut -d/ -f3) \
        $(echo "${date_from}" | cut -d/ -f2) \
        $(echo "${date_from}" | cut -d/ -f1) \
      )
))

(If you have the years, months and days of both dates in separate variables already the cut nightmare is not needed, obviously.)

Example:

$ ./diff_dates.sh 08/05/2013 01/01/1939
27156

Let's verify correctness with GNU date:

$ echo $(( ( $(date -d 2013-05-08 +%s) - $(date -d 1939-01-01 +%s) + 43200 ) / 86400))
27156